The present invention relates to a light amount control device provided within a copying apparatus and, more particularly, to an exposure amount control arrangement for use in a microfilm reader printer for controlling the amount of exposure on a projecting surface to a predetermined value in accordance with the output of the photo detector provided in the light path.
Generally, in microfilm reader printers, it is desirable that the amount of exposure in a process of exposure of images contained on a microfilm to light for producing projected images is controlled so as not to vary due to external factors such as variations in voltage, etc., except for the case where an operator changes the quantity of light for adjustment of density of images.
Meanwhile, since microfilms are standardized in various ways, they have different reduction ratios. Accordingly, in the case where images contained on such microfilms having various reduction ratios are projected for enlargement by a single reader printer, projection lenses having various magnifications corresponding to respective reduction ratios of the microfilms are interchangeably provided and thus, lenses are interchanged when microfilms having different reduction ratios are used.
Accordingly, since, in conventional reader printers, it has been so arranged that illuminance on a projected surface varies in response to changes in the magnification of lenses, the amount of exposure has been required to be controlled so that it will be maintained constant. Therefore, there have heretofore proposed or provided various methods or arrangements for automatic control of the amount of exposure in which photo detectors or photosensitive elements are provided on a projecting surface so that the amount of exposure may be controlled in accordance with the quantity of light received by the photo detectors.
However, the known arrangement as described above has such disadvantages that there are required not only a mechanism for retracting the photo detectors from the projecting surface at the time of exposure, but also a large number of photo detectors each having a large area for receiving light from enlarged, projected images.
In order to overcome the disadvantages as described above, there has also been proposed an arrangement in which a half mirror or the like provided in a light path directs light to a photo detector. But this arrangement, however, is also disadvantageous in that quantity of light received by the photo detector is not equal to that incident on the projecting surface. Namely, even if the illuminance of the light source is compensated at a predetermined rate corresponding to the lens being used so as to keep the amount of exposure on the projecting surface constant, the output of the photo detector has a different value, thereby posing a serious problem for automatic control of amount of exposure.